How Fluid Properties Affect Hydraulically Balanced Metering Pumps

By Donald B. Cameron, Pulsafeeder, Inc.

From Fluid Handling

Diaphragm metering pumps
that are hydraulically rather
than mechanically actuated
offer numerous attractions.
Their accuracy is high (typically
around 1%), and they can operate leakfree
even under high pressures (around
5,000 psi). And, they have rightly
gained a reputation for being able to
handle slurries and corrosive fluids.*

In spite of this versatility, optimal
specification and use of these pumps
requires attention to a broad range of
properties of the pumped fluid. These
include vapor pressure, viscosity, rheology,
temperature, corrosiveness, solids
content and specific gravity.

Vapor pressureAs with other pumps, the vapor pressure
of the fluid being pumped is important
because it helps establish the
available net positive suction head
(NPSH) at the pump inlet. To assure
proper delivery of the fluid to the inlet,
the available NPSH must be equal to or
greater than the manufacturer's stated
requirement for the pump. Otherwise,
the resulting cavitation can cause the
pump to lose metering accuracy or even
the ability to move the fluid at all.

Determination of the available
NPSH for a metering pump is more involved
than that for a centrifugal
pump. The metering-pump equation is:

NPSHa = Pa + Ph - Pv - LRGQ/C1 d2

where NPSHa is available net positive
suction head expressed in pressure
terms, Pa is the pressure (usually atmospheric)
on the surface of the fluid in
the supply tank, Ph is the elevation of
the fluid supply relative to the centerline
of the pump inlet, Pv is the vapor
pressure of the fluid at its flow temperature,
L is the length of the line connecting
the supply tank with the pump,
R is the pump's stroking rate (in cycles
per unit of time), Q is the volumetric
flowrate through the supply line, G is
the fluid's specific gravity, d is the pipe
inside diameter and C1 is a constant,
specified by the pump manufacturer.
The C1 term takes into account an average
roughness for the pipe, as well as
a typical number of pipe fittings.

Diaphragm metering pumps
that are hydraulically rather
than mechanically actuated
offer numerous attractions.
Their accuracy is high (typically
around 1%), and they can operate leakfree
even under high pressures (around
5,000 psi). And, they have rightly
gained a reputation for being able to
handle slurries and corrosive fluids.*

In spite of this versatility, optimal
specification and use of these pumps
requires attention to a broad range of
properties of the pumped fluid. These
include vapor pressure, viscosity, rheology,
temperature, corrosiveness, solids
content and specific gravity.

Vapor pressureAs with other pumps, the vapor pressure
of the fluid being pumped is important
because it helps establish the
available net positive suction head
(NPSH) at the pump inlet. To assure
proper delivery of the fluid to the inlet,
the available NPSH must be equal to or
greater than the manufacturer's stated
requirement for the pump. Otherwise,
the resulting cavitation can cause the
pump to lose metering accuracy or even
the ability to move the fluid at all.

Determination of the available
NPSH for a metering pump is more involved
than that for a centrifugal
pump. The metering-pump equation is: